Abstract
Coronal holes are usually defined as dark structures seen in the extreme ultraviolet and X-ray spectrum which are generally associated with open magnetic fields. Deriving reliably the coronal hole boundary is of high interest, as its area, underlying magnetic field, and other properties give important hints as regards high speed solar wind acceleration processes and compression regions arriving at Earth. In this study we present a new threshold-based extraction method, which incorporates the intensity gradient along the coronal hole boundary, which is implemented as a user-friendly SSW-IDL GUI. The Collection of Analysis Tools for Coronal Holes (CATCH) enables the user to download data, perform guided coronal hole extraction and analyze the underlying photospheric magnetic field. We use CATCH to analyze non-polar coronal holes during the SDO-era, based on 193 Å filtergrams taken by the Atmospheric Imaging Assembly (AIA) and magnetograms taken by the Heliospheric and Magnetic Imager (HMI), both on board the Solar Dynamics Observatory (SDO). Between 2010 and 2019 we investigate 707 coronal holes that are located close to the central meridian. We find coronal holes distributed across latitudes of about {pm}, 60^{circ}, for which we derive sizes between 1.6 times 10^{9} and 1.8 times 10^{11}mbox{ km}^{2}. The absolute value of the mean signed magnetic field strength tends towards an average of 2.9pm 1.9 G. As far as the abundance and size of coronal holes is concerned, we find no distinct trend towards the northern or southern hemisphere. We find that variations in local and global conditions may significantly change the threshold needed for reliable coronal hole extraction and thus, we can highlight the importance of individually assessing and extracting coronal holes.
Highlights
Coronal holes (CHs) are large-scale features in the solar corona often characterized by reduced emission in X-ray and extreme ultraviolet (EUV) which are associated with open magnetic field lines of a dominant polarity
We find a dependence on the area which seems to have two causes: (i) the larger impact of stray light for smaller CHs which could partly be compensated by performing a point spread function (PSF) deconvolution before the CH extraction and (ii) the non-zero extent of the boundary layer whose area is growing linearly in contrast to the total CH area
In this comprehensive study we investigated the intensity gradient across the CH boundary to develop a new CH extraction method using an intensity-based threshold method and to estimate the uncertainties of the extracted CH boundaries
Summary
Coronal holes (CHs) are large-scale features in the solar corona often characterized by reduced emission in X-ray and extreme ultraviolet (EUV) which are associated with open magnetic field lines of a dominant polarity. CHs can be observed as dark structures in the EUV and X-ray emission (see e.g., Schwenn, 2006; Cranmer, 2002, 2009). To investigate the morphology and intensity of CHs as observed in EUV, as well as their underlying photospheric magnetic field, the identification and extraction of CH boundaries are key. New approaches like machine learning/neural networks (e.g., Illarionov and Tlatov, 2018) and extraction methods based on plasma properties (differential emission measure; Raymond and Doyle, 1981; Hahn, Landi, and Savin, 2011) are the topic of current research
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